Reading a hash from a file on disk [duplicate] - ruby

Question

This question already has answers here:
Saving hashes to file on Ruby
(3 answers)
Closed 5 years ago.
Here is a hash that I save to a file to later be read.
my_hash = {-1 => 20, -2 => 30, -3 => 40}
File.open("my_file.txt", "w") { |f| f.write my_hash }
#how it looks opening the text file
{-1 => 20, -2 => 30, -3 => 40}
When I go to read it, is where my problem is. (following code is separate from top)
my_hash = File.foreach("my_file.txt") { |f| print f }
p my_hash
#=> {-1 => 20, -2 => 30, -3 => 40}nil
that nil messes up the rest of my code..not sure how to get rid of if. Just for clarity the rest of the code...
back_up_hash = {-1 => 20}
if my_hash.nil?
my_hash = back_up_hash
end
That little nil always makes my_hash equal to back_up_hash. I need that .nil? just in case the file is doesn't have the hash, otherwise the problem just gets pushed further down.
I also tried to read (slurp?..it's a small file) the file like this....
my_hash = File.read("my_file.txt") { |f| print f }
p my_hash
=> "{-1 => 20, -2 => 30, -3 => 40}"
# not sure how to get it out of string form...and I have searched for it.

Answer 1

You could use the eval method on the string (source)
eval("{-1 => 20, -2 => 30, -3 => 40}")
=> {-1 => 20, -2 => 30, -3 => 40}

Answer 2

If you want to take a file on disk whose contents are {-1 => 20, -2 => 30, -3 => 40} and make a hash from it, you want:
hash_str = File.read('my_file.txt')
my_hash = eval(hash_str) # Treat a string like Ruby code and evaluate it
# or, as a one-liner
my_hash = eval(File.read('my_file.txt'))
What you were doing is reading in the file and printing it to the screen, one line at a time. The 'print' command does not transform the data, and the foreach method does not map the data it yields to your block into any result. This is why you get nil for your my_hash.
As I recommended in a comment, if you have a Ruby object (like a Hash) and you need to save it to disk and load it later, you may want to use the Marshal module (built into Ruby):
$ irb
irb(main):001:0> h = {-1 => 20, -2 => 30, -3 => 40}
#=> {-1=>20, -2=>30, -3=>40}
irb(main):002:0> File.open('test.marshal','wb'){ |f| Marshal.dump(h, f) }
#=> #<File:test.marshal (closed)>
$ irb # later, a new irb session with no knowledge of h
irb(main):001:0> h = File.open('test.marshal'){ |f| Marshal.load(f) }
#=> {-1=>20, -2=>30, -3=>40}

Answer 3

The proper way to save simple data structures to a file is to serialize them. In this particular case, using JSON is probably a good choice:
# save hash to file:
f.write MultiJson.dump(my_hash)
# load it back:
p MultiJson.load(file_contents)
Keep in mind that JSON is only able to serialize simple, built-in data types (strings, numbers, arrays, hashes and the like). You will not be able to serialize and deserialize custom objects this way without some additional work.
If you don't have MultiJson, try it with JSON instead.

Answer 4

I have had success with these 2 simple methods:
def create_json_copy
File.open("db/json_records/stuff.json","w") do |f|
f.write("#{#existing_data.to_json}")
end
end
def read_json_copy
#json = JSON.parse(File.read("db/json_records/stuff.json")).as_json.with_indifferent_access
#json.each do |identifier,record|
existing_record = Something.find_by(some_column: identifier)
if !existing_record
Something.create!(record.except(:id).except(:created_at).except(:updated_at))
end
end
end
note: #existing_data is a Ruby Hash organised as { some_identifier: record_objet, ... } . I call .to_json on it before writing it to file and then when reading I JSON.parse it followed by .as_json, with_indifferent_access isn't really needed here so you can take it off as long as you substitute the symbols inside the excepts.